NASA tests optical links from space

Last Friday, NASA launched LADEE (“laddie”) a robotic probe that will orbit the moon and collect atmospheric information. What is particularly interesting to me about this probe is that it will also test communications with the earth over wireless optical links at 600Mb/s. (I first heard about this from an article by David Talbot on MIT’s Technology Review). So, I decided to dig deeper.

There are several really cool features to this spacecraft from a test/measurement perspective. According to NASA, the LADEE spacecraft is equipped with the following instruments to examine the atmospheric conditions of the moon: a UV and visible light spectrometer and a neutral mass spectrometer. Another purpose of this mission is a “technology demonstration” and that’s where the optical communications come in. Traditionally, probes like the LADEE only use a radio transmitter onboard and a huge satellite dish on the ground to communicate. While it will still be equipped with a traditional radio communications system, LADEE will also use lasers to communicate with Earth stations. I’m very interested to see how this test plays out.

I was fortunate enough to get to ask Dr. Donald Cornwell, Lunar Laser Communications Demonstration (LLCD) Mission Manager at NASA, a few questions about the optical technology. He confirmed for me that LADEE still has a traditional S-band radio transponder on board with a maximum downlink data rate of 128 kb/s. The actual experiments and operation of the LLCD will begin once the spacecraft reaches the moon and is in a stable orbit, probably in late October. Cornwell promises there will be more information in early November, and a technical presentation will occur on the topic at SPIE’s Photonics West in January.

Figure 1: This image shows the moon, Earth's only natural satellite, at
center with the limb of Earth near the bottom transitioning into the
orange-colored troposphere, the lowest and most dense portion of the
Earth's atmosphere. The troposphere ends abruptly at the tropopause,
which appears in the image as the sharp boundary between the orange and
blue-colored atmosphere. Image Credit: NASA

I asked him how the quality of transmissions will be tested, and he told me that they have a pseudo-random noise (PN) code generator onboard LADEE that can generate the 622 Mb/s data stream for the downlink form the Moon to the Earth. They also have a 40 Mb/s internal connection to the LADEE science buffer, and Cornwell says they hope to download science data for the mission (as a best effort, not baseline). He points out that since LADEE does not include a camera or other instruments that would generate large volumes of data, they included the 622 Mbps PN-code generator to create test data.

Once the data has been received on the ground, the ground teams will measure the error rate of the data as compared against the known PN-code. They also have a 20Mb/s laser uplink to LADEE, which will allow them to loop back the data.

Cornwell acknowledged the significant contributions of another member of the LLCD team, principal investigator Dr. Don M. Boroson from MIT’s Lincoln Laboratory (LL). The MIT LL designed, developed, integrated, tested, and delivered the space terminal and the primary ground terminal (at White Sands, NM) to NASA’s Goddard Space Flight Center. In addition, the NASA Jet Propulsion Lab (JPL) and the European Space Agency (ESA) are providing two ground terminals to provide geographic diversity in case there are dense clouds at any of the sites.

According to NASA’s launch materials, the LLCD payload system includes a laser mounted on an articulated boom. Several earth facilities are supporting LADEE including, the Multi-Mission Operations Center (MMOC) at NASA's Ames Research Center in Silicon Valley, CA, which will serve as a base for mission operations and real-time control of the observatory. In addition, the spacecraft has a Science Operations Center at NASA's Goddard Space Flight Center in Greenbelt, MD, that will catalogue and distribute instrument data to the science team located in instrument operations centers. There will be several radio communication facilities to communicate with the spacecraft; the primary ground station is an 18-meter antenna located at the White Sands Complex near Las Cruces, N.M. Other facilities include the Deep Space Network's 34-meter antennas located in Madrid, Spain and Canberra, Australia.

LADEE should take about 40 days to get set up, and then perform about 100 days of science data collection. After that, the spacecraft will impact on the moon.

Figure 3: LADEE’s end of life, as conceptualized by an artist for NASA

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A paper on how optical satellite networks work by Nikos Karafolas and Stefano Baroni, "Optical Satellite Networks" in the Journal of Lightwave Technology. Karafolas and Baroni explain that these networks can multigigabit laser intersatellite links (ISLs) in order to route traffic through space and create a global space-based optical backbone network.